As mobile device technology continues to develop and demand therefor continues to increase, demand for secondary batteries as energy sources is rapidly increasing. Among these secondary batteries, research on lithium secondary batteries, which exhibit high energy density and discharge voltage, has been underway and such lithium secondary batteries are commercially available and widely used.
Secondary batteries are classified into a cylindrical or rectangular type battery including an electrode assembly in a cylindrical or rectangular metal can and a pouch type battery including an electrode assembly in a pouch type case made of an aluminum laminate sheet according to shapes of battery cases. Among these batteries, a cylindrical type battery has a relatively high capacity and structural stability.
An electrode assembly accommodated in a battery case is a rechargeable power generation device and has a stacked structure consisting of a cathode, a separator, and an anode, and is classified into a jelly-roll type electrode assembly fabricated by interposing a separator between a cathode having a long sheet shape and coated with an active material and an anode and a stacked type electrode assembly fabricated by sequentially stacking a plurality of cathodes and anodes having predetermined sizes with separators disposed therebetween. Of these, the jelly-roll type electrode assembly is easily manufactured and has high energy density per unit weight.
With regards thereto, FIG. 1 is a vertical sectional perspective view of a general cylindrical battery 100.
Referring to FIG. 1, the cylindrical battery 100 is fabricated by placing a jelly-roll type (winding type) electrode assembly 120 in a cylindrical case 130, injecting an electrolyte into the cylindrical case 130, and connecting a top cap 140 provided with an electrode terminal (e.g., a cathode terminal, not shown) to an opening top of the cylindrical case 130.
The electrode assembly 120 has a structure in which a cathode 121, an anode 122, and a separator 123 disposed therebetween are rolled in a circular form and a center pin 150 of a cylindrical type is inserted into a winding core thereof (the center of the jelly-roll). The center pin 150 is generally made of a metal material to impart predetermined strength and has a hollow cylindrical structure formed by bending a plate in a circular form. The center pin 150 fixes and supports the electrode assembly 120 and serves as a passage through which gases generated by internal reaction during charge/discharge and operation of a battery are discharged.
Meanwhile, lithium secondary batteries have low safety. For example, when a lithium secondary battery is overcharged to approximately 4.5 V or higher, a cathode active material is decomposed, dendrite growth of lithium metal at an anode occurs, and an electrolyte solution is decomposed. Such processes involve heat and thus the above-described decomposition reactions and various side reactions rapidly proceed, which eventually leads to combustion and explosion of a battery.
Thus, to address these problems, a general cylindrical battery includes a current interrupt device (CID) and a safety vent in a space between the electrode assembly 120 and the top cap 140. In this regard, the CID and the safety vent serve to interrupt current and reduce internal pressure of a battery when the battery abnormally operates.
In particular, referring to FIG. 2, a top cap 10 forms a cathode terminal in the form of a protrusion and is provided with a perforated vent. A positive temperature coefficient (PTC) element 20 that is disposed below the top cap 10 and greatly increases battery resistance and thereby interrupts current when an internal temperature of the battery increases, a safety vent 30, which protrudes downward in a normal state and protrudes upward as an internal pressure of a battery increases to eventually explode so as to safely discharge gases, and a connection plate 50, one side of the top of which is connected to the safety vent 30 and the other side of the bottom of which is connected to the cathode of the electrode assembly 40, are arranged under the top cap 10 in this order.
Accordingly, the cathode of the electrode assembly 40 is connected through a lead 42, the connection plate 50, the safety vent 30, and the PTC element 20 to the top cap 10 under normal operation, to supply electricity.
However, secondary batteries for power tools are operated under poor environments when compared to other batteries used under different environments and thus it is the most important to minimize heat generation by reducing internal resistance of batteries. To address these problems, conventionally, a method of increasing the size of a cathode lead or using two anode leads is used.
In particular, an existing anode lead is made of a single material such as nickel and has a single size. In addition, a material having a lower resistance than that of nickel, such as copper may be used as the anode lead, but is not suitable because it is difficult to secure processability for welding to an electrode foil or can.
Therefore, there is an urgent need to develop a technology for secondary batteries that can fundamentally address these problems and reduce resistance and heat generation while secondary batteries are used.